InstallMate sets this variable to the installer's exit code. See InstallMate Command Line Options for a full list of possible exit codes.
You can refer to this variable as <TsuExitCode>. In conditional expressions you can omit the brackets and simply use TsuExitCode as a test for failed execution or NOT TsuExitCode for success. These are equivalent to the more explicit tests TsuExitCode <> 0 and TsuExitCode = 0, respectively. Do not change its value; it is set and updated automatically by InstallMate.